Bug 28938: Correct Flatpickr's default 12hr time formatting

Bug 28938: Correct Flatpickr's default 12hr time formatting

This patch corrects Flatpickr's default 12 time formatting so that it
uses the formatting token "G" ("Hours, 2 digits with leading zeros")
instead of "h" ("Hours, 2 digits without leading zeros").

This will prevent incorrect times from being saved when the submitted
time is before 12.

- To test, apply the patch go to Administration -> System preferences.
- Set the TimeFormat system preference to "12 hour"
- Find an item which is checked out and renewable.
- Go to Circulation -> Renew and use the date picker to select a time <
  12, e.g. 9:00 AM
- Submit the barcode for renewal.
- Check the patron's account to see the due date of the renewed item: It
  should match the date and time you selected.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>